I recommend you upgrade the switch !!!
Below the releases that should solve your problems:
A5500EI-CMW520-R2221 Changes in Operations 1. Changed the maximum ARP rate on a port that is enabled with ARP detection from 50 pps to 400 pps.
A5500EI-CMW520-R2221P20
201510200101
Symptom: The switch prints a message indicating an IP address conflict when a newly online wireless client obtains the IP address of a wireless client that just went offline.
Condition: This symptom occurs if the following conditions exist:
Wireless clients obtain IP addresses through DHCP.
A wireless client comes online after another wireless client goes offline.